This invention pertains to solids pulverizer mills having multiple air port nozzles oriented for flow stream interaction and control of acoustic resonance vibrations in the mill. It pertains particularly to a coal pulverizing mill and process providing multiple intersecting and interactive air jet streams for effectively entraining the pulverized coal upwardly without causing acoustic resonance vibrations in the mill.
Pulverizer mills such as used for grinding raw coal to small particle size for feed to combustion furnaces are well known. For example, U.S. Pat. No. 3,465,971 to Dalenberg et al discloses a coal pulverizing mill with stationary deflector vanes positioned above the grinding ring for directing airborne pulverized material downwardly and inwardly back towards the grinding ring. U.S. Pat. No. 4,234,132 to Maliszewski discloses a pulverizing mill having stationary air deflector means located above the rotary grinding table. U.S. Pat. No. 4,602,745 to Maliszewski et al discloses a bowl pulverizer mill having primary classifier containing vanes located above the grinding table and consisting of converging/diverging orifice means. U.S. Pat. No. 4,602,745 to Provost discloses a pulverizer mill having a circumferential throat ring containing a plurality of angularly disposed stationary air channels through which air is forced upwardly in contact with the pulverized coal causing it to be entrained upwardly. U.S. Pat. No. 5,020,734 to Novotny et al discloses a pulverizer having a rotary table with multiple angled replaceable air ports. Also, U.S. Pat. No. 5,090,631 to Wark discloses a pulverizer mill having adjustable deflector air flow rate control means provided around the rotatable table. However, none of the known prior art is directed to use of multiple air ports providing angled intersecting air jet streams which are sufficiently interacting to produce intense turbulence, and may be adjustable so as to reduce or eliminate acoustic resonant vibrations of the entrained air-coal mixture mass in a pulverizer mill.
Coal pulverizer mills grind coal typically from 0.5 to 2 inch size pieces to provide fine coal dust particles usually less than a micron up to several microns in size. The grinding is accomplished by multiple grinding or pulverizing rollers rotating about their own axes and crushing the coal against a rotating table driven by a motor through a speed reducer. The rotation of the table induces rotation of the rollers which are pressed downwardly either by springs or by hydraulic or pneumatic means toward the rotary table to enhance the coal crushing and pulverizing action. The raw coal feed enters the mill vertically by gravity and the ground pulverized coal is carried from the mill by air entrainment upwardly through a classifier section to external burners for combustion. The mill classifier allows the fine enough particles to pass on to the external burners, while the coarser size particles are returned to the mill rotary table for further grinding and size reduction.
A substantial air flow rate is needed to carry the pulverized coal from the mill table upwardly through the classifier and to the burners. The air/coal weight ratio needed for pulverization, coal particle transport and combustion is in the range of 1.5/1 to 3.5/1, depending on coal type and flow rate. The air enters the mill plenum located beneath the grinding table, and enters the grinding section through multiple air ports which are typically evenly spaced around the grinding table circumference. Many air ports are used, typically in the range of 16 to 40, depending on the mill size and the type of coal being ground. The air ports are usually stationery (non-rotating) and attached to the mill housing, but can be rotary type attached to the rotatable grinding table.
The air exits from the air ports as high velocity air jet streams which are typically provided parallel to each other and have a forward angle relative to the plane of the table which is typically at 30.degree.-45.degree. angle, but other angles may be used. The air jets generate a swirling action for the entrained coal particles, the orientation of which is preferably in the same direction as the table rotation, for reasons of good performance of the pulverizer.
Pulverizer mill operation experience has shown that the issuing air jet action can cause undesirable acoustic resonance and vibrations inside the pulverizer mill housing. The driving excitation vibration is generated by the air jet swirling action, and is accompanied by a corresponding pressure pulsation representing a forcing function which excites the acoustic vibration. The acoustic resonance occurs when the excitation frequency generated by the air jet streams coincides with one of the acoustic (natural) frequencies of the air or air/coal mixture inside the mill. Coincidence of the air jet excitation frequency with the fundamental acoustic or natural frequency (1st mode) typically generates the most severe resonance, leading to large acoustic pressures inside the mill housing and resulting in severe structural vibrations of the mill. Coincidence of air jet excitation with higher natural frequency modes (2nd, 3rd, etc.) results typically in lower acoustic pressures inside the mill. Such vibration interferes with the normal operation of the pulverizer mill and also may produce structural damage, and cannot be tolerated.
The required air jet velocity in a pulverizer mill has lower limits, because a minimum air velocity is needed to entrain the coal upwardly from the rotary table and prevent it from falling back down through the air port openings into the air plenum. This minimum air jet velocity is a function of the coal particle size and weight. For a coal particle size of about 1.5 inch, and air temperature of about 450.degree. F., the minimum required jet velocity is approximately 150 ft/sec which velocity prevents the coal particles from falling back down through the air ports. For the reasons explained above, lowering the air jet velocities in order to avoid acoustic resonance vibrations becomes impractical. To avoid acoustic resonance conditions, increasing the air jet velocity in conjunction with a reduction in the jet streams intersection angle remains the only viable option. However, there are two problems with increasing the air jet velocity to avoid resonance within the entire operating range of the mill. The air jet velocities must be quite high (in the 300-400 ft/sec range) for avoiding acoustic resonance within the entire range of air velocities and coal particle flows, and such high velocities may detrimentally affect mill performance and increase mill erosion. Also, such high air jet velocities would generate undesirable pressure drop across a pulverizer mill, thereby reducing mill and fan efficiency.
Even if acoustic frequency separation is achieved by changing the pulverizer mill coal flow load and thereby changing air flow for optimum mill performance, the frequency separation may be reduced to the point that the pulverizer mill would become sensitive to acoustic resonance. If the frequency separation becomes insufficient, the mill may commence vibrating. Once a mill starts vibrating, it will continue to vibrate through a large range of air flow velocities due to the well-known lock-in phenomenon. Only a significant change in air flow and/or coal flow will interrupt the pulverizer mill vibratory condition. Thus, it can be seen that the solution to the acoustic resonance vibrations by way of separation of acoustic frequencies is not a desirable or viable solution in most cases, so that other remedies have been sought.